Anodic coating of aluminum



1" eral characteristics of a coatin Patented July 26, 1932 UNITED STATES PATENT OFFICE onaawa noon aoraa'rs down, or LONDON,

ENGLAND; ABSIGNOB TO ALUIINUI corona mcoaroaa'ran, or mnumarons, ncurs, a coaroaarron' or nmuxa moms comma or arum'mum 7 Ho Drawing. Application filed October 29, 1931, Serial No. 571,945, and in Great Britain October 80, 1887.

This invention relates to anodic coating of aluminum; and it comprises an article of aluminum or aluminum alloy carrying a coating of substantial thickness having the genproduced by making such an article an ano e at low voltages in sulfuric acid of low concentration approximating 15 to 35 per cent by ,weight, this coating containing A1 S0 and H 0 10 in combination, being laminar with a flint-like ll process of Ions other operations such as rinsing,

underlayer and a somewhat softer outer layer, being insoluble, resistant to wear and abra sion, absor tive, porous and capable of being buffed an polished; and it also includes a coating aluminum articles which comprises immersing such articles in a bath of sulfuric acid of a low concentration approximating to 35 per cent by weight and assing an electric current at low voltages om such articles to other electrodes in said bath, this treatment being followed by varidyeing, bufling, po shing, oiling, pigmenting, etc; all as more fully hereinafter set forth and as claimed.

The present application is a continuation in part of my copending application Serial N 0. 253,790, filed February 11, 1928, and comprises certain extensions of the methods therein described as well as improvements in the articles produced. In my earlier application I described a process of coating aluminum articles by making the same anodes at low voltages in a bath of sulfuric acid of a concentration ranging from 1 to 2 pounds per gallon or approximately 15 to per cent of H SO. by weight.

I have now found that the upper limit of the concentration range can be extended up to 35 per cent sulfuric acid with some advantages, an optimum value apparently lying in the neighborhood of about per cent. At concentrations below 15 per cent there is a decided increase in cost due to a higher current consumption. A different ty e of coating appears to be formed at these ower concentrations which is not laminar, a feature which is a characteristic of my coatings. At concentrations above per cent the coatinlgs Y formed are not as adherent. They usua laminar structure distinguish t chip and craze. Consequently there is a rather narrow range of acld concentration over whlch the most advantageous coatings are obtained.

The coatings obtained over this particular acid range are characterized b their laminar structure which consists in a 'nt-like underlayer and a somewhat softer outer layer, the latter still bein sufliciently hard to withstand bufiing, a rasion, etc. Usin the socalled microcharacter scratch test or hardness, with a standard 3 gram load, one speci- -men, coated in a 25 per cent bath, was found to have a surface hardness of 140. The hardness next to the metal was found to be 5000. ThlS is 1n comparison with glass and with chromium having hardnesses of 2000 and 3100, respectively.

In spite of their hardness my coatings are firml coherent to the. underlying metal WlllC can be bent to a considerable extent without damaging the coating. Although hair line cracks are likely to appear this has been foun not torimpair the resistance to COII'OSIOII. 75

My coatings are minutely pervious, the pores being. sub-microscopic and having a volume amounting to about 25 per cent of the total volume of the coating. These pores are capable of absorbing and adsorbin various materials such as dyes, pigments, oi s, paints, etc. A treatment with such materials great,- ly improves the corrosion resistance by plugging the pores and making them impervious. The S0,, content ranges from about 12 to 16 per cent by weight and SO; appears to be a necessary component of my coatings. My coatings are insoluble in water and resistant to the acid of the coating bath.

The thickness of my coatin and their em clearly from the prior art, in which generall films were obtained of no substantial thiclmess and providing but slight protection from corrosion. Satisfactory coatings for my purposes range in thickness from about 0.0002 to 0.001 inch. In order to obtain coatings 0.0002 inch thick, a treatment in the sulfuric acid bath of only 10 minutes, at about 12 volts, is required. The thicker films require 1 a time of treatment ranging up to 45 minutes or an hour. Longer treatments do no harm but the thickness of the film is not substantially increased after an hour.

The voltages suitable for In process range from about 6 up to 20 volts. o-called addition agents can be added to the bath if desired. Ordinary room temperatures may be used, that is temperatures ranging from about 60 to 100 F. Practically any type of aluminum alloy is suitable, whether rolled or cast. The alloys should contain at least 85 per cent of aluminum. Some of the allo s produce colored or tinted coatings, but the usual color of the coating produced on substantially pure aluminum 1S pearlish cloudy white. Duralumin produces a ay coating, this color also resulting with a oys containin silicon or tin.

a specific embodiment of my invention I ma take an article of pure r led or cast aluminum and polish the same. The article should then be carefully cleaned b being immersed in a solvent for grease an thereafter rinsing. Castin 5 should be cleaned mechanically with an a rasive powder and brush to assure that all holes and cavities are free from dirt, prior to cleaning in the grease solvent. The cleaned article is then connected with a source of electrical potential and made the anode'in a bath comprising an aqueous solution of sulfuric acid of 25 per cent strength by weight, care being taken that the object is not put into the bath unless the electric current is on. The cathode may be of carbon or lead. The object may be suspended in the bath in electrical connection by thick aluminum wires or aluminum clamps, care being taken that the length of connecting conductor in the solution is as short as possible; or it may be suspended by wires of metals not attacked by the bath. In the case of big sheets, castings or objects in which a coating. of absolutel uniform appearance is not essential the ob ect may be partially immer'sed and treated and then inverted so that the remainder of the surface may be similarly treated, the electrical connections in each case being made outside the bath. The voltage across the bath may preferably be about 6 volts. With this voltage,when the object is first immersed the current rises to about 10xa1hperes per square foot which rapidly falls to a steady current of about 2' amperes per square foot. This procedure will give a satisfactory coating in about 25' minutes. The bath should be stirred or agitated before and during use. The bath should be brought back to strength from time to time by the adddition of the. correct amount of sulfuric ac1 After the anodic treatment, the article should be removed from the sulfuric acid bath and washed with water. It may then be dyed or, after drying, the pores may be In ged with oil pi ents, paint, lacquer nd the like. Bufling parts a highly glossy finish and the usual grease bufiing compounds also fill the pores.

What I claim is:

1. In the anodic coating of aluminum and aluminum alloy articles, the rocess which comprises treating such artic es anodically with low working voltages in a bath of sulfuric acid having a low concentration of about 25 per cent by weight.

2. In the anodic coating of aluminum and aluminum alloy articles, the rocess which comprises treating such artic es anodlcally with low working voltages in a bath of s ul-' furic acid having a concentration ranging from about to per cent by weight.

3. In the anodic coating of aluminum and aluminum alloy articles, the rocess which comprises treating such 'artic es anodically with low working voltages in a bath of sulfuric acid having a concentration ranging from more than 20 to 35 per centby weight.

4. In the anodic coatin of aluminum and aluminum alloy articles,t e process compris-' ing passing current from such articles to other electrodes in an electrolytic bath having a concentration of about 15 to 20 per cent sulfuric acid and with voltages ranging from 6 to 20 volts. 5. In the anodic coatin of aluminum and aluminum alloy articles, t e process comprising passing current from such articles to other electrodes in an electrolytic bath having a concentration of more than 20 to 35 per cent sulfuric acid and with voltages ranging from a to 20 volts. v 6. In the anodic coating of aluminum and aluminum alloy articles, the process which comprises anodically coating such articles in a bath of sulfuric acid having a cencentration ranging from about 15 to 20 per cent by weight and finall; plugging the pores of the coating so forme 7. In the anodic coating of aluminum and aluminum alloy articles, the process which comprises anodically coating such articles in a bath of sulfuric acid having aconcentration ranging from more than 20 to 35 per cent by weight and finall plugging the pores of the coating so formed 8. In the'anodic coating of aluminum and aluminum alloy articles, the process which comprises anodically coating such articles in a' bath of sulfuric acid having a concentration ranging from about 15 to 20 per cent by weight and finally filling the pores of said coating with oil.

9. In the anodic coating of aluminum and aluminum alloy articles, the process which comprises anodically coating such articles in a bath of sulfuric acid having a concentration ranging from more than 20 to 35 per cent by weight and finally filling the pores of said coating with oil; a

. 10. In the anodic coating of aluminum and aluminum alloy articles, the process which comprises anodlcally coating such articles in a bath of sulfuric acid-having a concentration I ranging .from about 15 to 20 per cent by weight and finall bufling said coating. 11. In the ano ic coating of aluminum and aluminum alloy articles, the process which comprises anodicall coating such articles in a bath of sulfuric acid having a concentration ranging from more thari 20 to 35 per cent by weight and finally bufiing said coating.

12. As a new article, a body of aluminum or aluminum alloy carrying a, coating of substantial *thibkness having the general characteristics of one produced by making such an article an anode at low voltages in a sulfuric acid bath of low concentration of 15 to 20 per cent by weight, this coating containing A1 0,, S0 and H 0 in combination, be-

ing laminar with a flint-like under layer and a somewhat softened outer layer, being insoluble, resistant to wear and. abrasion, absorptive and adsorptive, porous and capable of being buifed.

13. As a new article, a body of aluminumor aluminumalloy carrying a coating of substantial thickness having the general characteristics of one produced by ,makingfsuch an article an anode at low voltages in a sulfuric acid bath of low concentration of more than 20 to 35 per centby weight, this coating containing A1 0 S0 and H 0 in combination, being laminarwith a'flint-like under layer and a somewhat softened outer layer,

being insoluble, resistant to wear. and abrasion, absorptive and adsorptive, porous and capable of being bufied. 14. The article of claim 12 in which the .pores of said coating are-plugged, thereby rendering the said coating more resistant to corroslon. i

15. The article of claim 13 in which the pores of said coating are plugged, thereby rendering the said coating more resistant to corrosion. I i 16. The article of claim 12 in which the gores of said coating are filled with oil, there y rendering the said coating more resistant to corrosion.

17. The article of claim13 in which the gores of said coating a're filled with oil, therey rendering the sald coating more resistant to corrosion. 18. The article of claim 12 in which the said coating has been finished by buflin 19. The article of claim 13 1n whic the said coating has been finished by bufling.

In testimony whereof I have hereunto, w afiixed my signature.

f CHARLES HUGH] ROBERTS GOWBIL 

